This application claims the priority of German application 196 39 053.2, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a die casting machine having a divisible die casting tool and a device for opening and closing the die casting tool as well as having a device for pressing together mutually detachable tool parts during the casting operation against the spreading forces acting upon them on the part of the molten mass affected by pressure.
W. Bovensmann xe2x80x9cNeuzeitliche Druckgiegmaschinen fur prxc3xa4zise, groxcex2volumige Druckguxcex2teilexe2x80x9d (xe2x80x9cModern Die Casting Machines for Precise Large-Volume Die-Cast Partsxe2x80x9d) in the Journal Giexcex2erei-Praxis, No. 7/1966, Pages 115 to 119, describes only die-casting in detail, although since 1966 other similar-type casting processes have also been considered. In this connection, die-casting with an evacuated die-casting mold as well as thixocasting and thixoforming are known casting processes. Generally, these are deforming processes in which a metal quantity adapted to the workpiece size is charged into an openable and closable shaping tool made of metal. The mold is filled with the flowable light metal which is shaped under the effect of a high pressure. The workpiece is then caused to solidify under the effect of this pressure until the workpiece can be handled and is removed from the mold. Here, the term xe2x80x9cdie castingxe2x80x9d includes all known shaping processes which fall under this sub-definition.
In the above context, reference can, for example, also be made to the special edition of the journals ATZ and MTZ for the year 1996 concerning the topic xe2x80x9cLight Metals in Automobile Constructionxe2x80x9d containing the following-articles by B. Wendiger, A. Mundl: xe2x80x9cThixocastingxe2x80x94A Deforming Technique with a Futurexe2x80x9d; or by G. Springe: xe2x80x9cThe Future of the Use of Aluminum in Automobile Constructionxe2x80x9d.
In principle, the known die casting machines are built like a horizontal press with two opposed, vertically standing clamping plates, of which one can be driven in a stationary and the other can be driven in a liftable manner. By way of four tie bars, the movable clamping plate is guided parallel to the stationarily held clamping plate. On the mutually facing sides of the clamping plates, the respective parts of the die casting tool are clamped and are divided along a vertical mold parting plane. This arrangement is required because of a standing casting position. The mold must fill in an ascending manner in the direction of the gravitational force.
As above mentioned, during the casting and the subsequent setting, a high pressure is exercised on the molten mass which endeavors to drive apart the parts of the die casting tool. The level of the spreading force is the result of the level of the pressure and of the projected area of the metal-wetted surfaces of the die casting tool. The die casting tool must be held in the closed condition by a clearly higher force so that the tool parts will not open, not even slightly, and no casting burr can form at the edges. The required closing forces are transferred by way of the tie bars and the clamping plates to the parts of the die casting tool. A hydraulic cylinder acts in the closing direction onto the movable clamping plate and is supported on a crosshead which is rigidly connected via the tie bars with the stationary clamping plate.
In known die casting machines, the size of the castings which can be safely cast in the process is limited by the elasticity of the clamping plates. Under the load of the closing forces which are applied on the edge side by way of the tie bars to the stationary clamping plate, on one hand, and the pressure forces of the die casting tool which act upon it essentially in the center, on the other hand, because of Hook""s law, a slight but nevertheless unavoidable deformation of the clamping plates and of the die casting tools carried thereby will occur. The increasing size of the die casting tools causes not only an intensification of the effective forces, specifically with the second power of the linear dimension, but the bends will also increase, specifically with the third power of the linear dimension. With a 10% enlargement of the clearance between the tie bars and with a 10% enlargement of the lateral length of the die-cast part, while the conditions are otherwise comparable, the occurring forces are approximately 21% larger, and the bends, at 100% of the forces, are approximately 33% larger, and at 121% of the forces are approximately the sum of these increases, thus approximately 54% larger. With a 20% enlargement of the linear dimension of a die casting machine, the bends increase approximately to twice the size while the conditions are otherwise comparable. Although the bend can be compensated by suitable stiffening measures on the clamping plates, these measures are only partly effective because the laws of nature (here, Hook""s law) cannot be canceled by constructional techniques. A remaining bend will always remain and it is at most a question of whether the extent of the bend can be tolerated. Starting at a certain span between the tie bars, even stiffening measures will no longer be effective for keeping the bending of the clamping plates within tolerable limits. As a result of the elasticity of the building material, at least in the situation of this size of die casting machines, a natural limit is reached for the workpiece size.
An object of the present invention is to improve the known die casting machine such that, beyond the indicated, elasticity-caused limits of a die-cast part size, die-cast parts can be cast in a reliable manner according to the process, i.e., with only slight and easily tolerable elastic deformations of the machine parts and tool parts after the die casting process.
This object has been achieved according to the present invention by providing a directly mutual, openable and closable, form-locking clamping-together of the mutually detachable tool parts of the die casting tool which transmits the closing pressures counteracting the spreading forces in a power flux circuit remaining within the die casting tool. Accordingly, the power flux circuit for holding together the parts of the die casting tool are short-circuited within the die casting tool so that the clamping plates are no longer loaded by the closing forces.
The tool parts clamped onto the clamping plates benefit fully with respect to the stiffness from the stiffness of the clamping plates. With respect to their position and their mutual distance, the tool-integrated tie rods for holding together the die casting tools can be adapted individually for each workpiece to the respective die-cast part which results in optimally short bending lengths therebetween. Mainly, one or several tie rods can be placed in the interior of the outer contour of the die-cast part, specifically if, as normally, the die-cast part has open areas in the manner of a window-type breakthrough, a framed compartment, or the like.
The present invention offers advantages not only with respect to a reliable manufacturing of previously uncontrollable large die-cast parts but can advantageously also be used with conventional size die casting machines because such machines can easily be constructed lighter and therefore lower in cost.
In order to be able to limit the flow paths of the molten mass to sections within large die casting tools which can be covered by the molten mass scattered into small droplets without any excessive cooling, an advantageous embodiment of the invention provides several distributed casting cylinders for feeding material for casting into the casting mold so that the casting mold can be filled with the molten mass simultaneously from several points.